Micro-machined components formed in a silicon device layer of a semiconductor wafer, in general, are formed in a relatively thin silicon device layer, which is supported on a handle layer. The device layer in which the micro-machined components are to be formed is laminated to the handle layer, which, in general, is also of silicon. In general, an oxide layer is formed between the handle layer and the device layer. The handle layer provides support to the relatively thin device layer within which the micro-machined components are formed. The oxide layer forms an electrical insulation barrier between the device layer and the handle layer. In general, it is necessary to be able to access such micro-machined components through the handle layer, and this requires the formation of access bores extending through the handle layer to the respective micro-machined components. In general, it is desirable that the access bores to such micro-machined components should be accurately aligned with the corresponding one of the micro-machined components, and additionally, it is desirable that the access bores should be accurately dimensioned, and in particular, the transverse cross-sectional area of such bores should be of accurate dimensions. For example, where it is desired to terminate an optical fibre extending through an access bore adjacent the corresponding micro-machined component, it is important that as well as being accurately aligned with the micro-machined component, the access bore should be accurately dimensioned in order to positively and accurately secure and locate the optical fibre relative to the micro-machined component. It is also desirable that such access bores be dimensioned to form a relatively tight fit around to the corresponding optical fibre in order that when the optical fibre is tightly located in the access bore, the terminal end of the optical fibre is accurately aligned with the micro-machined component. In general, axial alignment of such access bores relative to the corresponding micro-machined component can be achieved without too much difficulty. However, the etching of such access bores of relatively accurate dimensions, particularly relatively accurate cross-sectional dimensions, presents considerable difficulties, and thus subsequent alignment problems when locating the optical fibre in the access bore relative to the micro-machined component.
Additionally, due to the fact that the optical fibre should be a relatively tight fit, and preferably, an interference fit in the access bore, it is desirable that a tapered lead in should be provided to the bore for facilitating initial insertion of the optical fibre into the access bore. This also is difficult to achieve with any degree of accuracy.
In known methods for forming such access bores, an anisotropic wet etch is used where the etchant may, for example, comprise a mixture of potassium hydroxide, isopropylalcohol and water. In general, it is difficult to control the cross-sectional shape of an access bore in such wet etch processes. In particular, it is difficult to wet etch such access bores of regular circular cross-section. This is due to the fact that wet etches tend to etch along the crystalline plane of silicon, and typically, attempts to etch bores of circular cross-section tend to result in bores of square or rectangular cross-section. This is so irrespective of the etch opening formed in a mask through which the etchant is being directed at the silicon.
There is therefore a need for a method for etching a bore, and in particular, a tapered bore into a silicon substrate which overcomes these problems.
The present invention is directed towards providing such a method, and the invention is also directed towards providing a semiconductor wafer comprising a substrate layer having a bore etched therein by the method according to the invention.